The present invention relates generally to a method of forming an asymmetric extension MOSFET (metal oxide semiconductor field effect transistor) using a drain side spacer, and more particularly to a method of forming an asymmetric extension MOSFET using a drain side spacer which allows a choice of source and drain sides for each individual MOSFET device and also allows an independent design or tuning of the source and drain extension implant dose as well as its spacing from the gate.
Asymmetric MOSFET devices are known MOSFET devices wherein the source and drain regions are not symmetrically doped because of different implant dosages or because of asymmetric source and drain extension implant relative to the gate channel conductor.
Asymmetric MOSFET devices are known to provide advantages of increased drive currents and reduced parities. Asymmetric extension and halo devices can be fabricated by using angled implants and by using the gate to mask the source or drain region. This prior art method of fabrication does not allow independent definition of the source and drain regions for each device which is critical for efficient circuit design. Also, using angled implants and using thick photoresist shading on the drain or source side is not very desirable for fabricating devices with an asymmetric extension.
The present invention provides a method of forming an asymmetric extension MOSFET by using a drain side spacer which allows a choice of source and drain sides for each individual MOSFET device and also allows an independent design or tuning of the source and drain extension implant dose as well as its spacing from the gate.
The method starts with a semiconductor device produced by art known techniques and having a substrate on which gate conductors separate source regions and drain regions, with an oxide layer formed over the gate conductors, the source regions and the drain regions, and a nitride layer formed over the oxide layer. A photoresist mask is formed over at least a portion of each drain region, followed by an angled ion implant during which the photoresist mask and the gate conductor shield the nitride layer over at least a portion of the drain region and on at least one sidewall of the gate conductor from damage by the angled ion implant which selectively damages portions of the nitride layer unprotected by the photoresist mask and the gate conductor. Then damaged portions of the nitride layer are removed while leaving undamaged portions of the nitride layer as a nitride mask to protect at least a portion of each drain region and at least one gate sidewall from a subsequent dopant implant which is performed into the source regions and the drain regions while using the undamaged portions of the nitride layer as a mask to form the asymmetric extension MOSFET device.
The present invention allows a drain region to be defined on either side of the device and provides a spacer on the drain side of the device.